The durability and flame retardancy of cotton fabrics have been the focus of long-term research. In this paper, a method for preparing flame retardants through the direct modification of biomass was proposed, and the durable flame retardant of homologous cottonseed meal modified biomass flame retardants for cotton fabrics was achieved through biomass composition analysis and modeling. In this study, a cottonseed meal-phosphoric acid-boric acid synergistic bio-based flame retardant (CPB) was synthesized and characterized. The results of XPS and scanning electron microscope (SEM) showed that DC-CPB1 was mainly grafted on the superficial layer and surface of the fabric, and DC-CPB2 was mainly grafted on the surface of cotton fiber. In the vertical flammability test (VFT) and limiting oxygen index test (LOI), the LOI value of cotton fabric after 50 cycles of washing can reach up to 33.5 %, and the carbon damage length was reduced from 300 mm to a minimum of 69 mm, showing excellent flame-retardant performance and self-extinguishing ability. In the cone calorimetry test, the peak heat release rate (PHRR) was significantly reduced by 82.49 %, and the results of thermogravimetric (TG) analysis and thermogravimetry-infrared-mass (TG-IR-MS) test showed that CPB1 mainly had a gas phase. In contrast, CPB2 mainly had a condensed phase flame retardant mechanism. The mechanical properties of the cotton fabric remain good after treatment.